18 Coronary Arteries and Myocardial Perfusion



Farhood Saremi

18 Coronary Arteries and Myocardial Perfusion



Introduction


Knowledge of gross anatomy and microcirculation of the coronary arterial system is important not only to address the spatial location of a lesion but also to understand the mechanisms that lead to myocardial dysfunction. The standard clinical tool to visualize the coronary artery has been invasive cardiac catheterization and coronary angiography. Catheter angiography images have higher spatial resolution compared to recently developed noninvasive coronary computed tomography angiography (CTA). The anatomy of the epicardial coronary arteries can be shown down to third-generation (medium- to small-sized) branches by catheter angiography and to second-generation (medium-sized) branches by CTA. As opposed to projectional views in catheter angiography, CT imaging has solved the problem of overlapping vessels by generating a volume of data consisting of 0.5 mm3 voxels that can be reconstructed into 0.5-mm cuts in any desired direction. Although, the spatial and temporal resolutions of CT are lower than in invasive angiography, the noninvasive nature of the test has led to a rapidly increasing use in clinical practice. Using modern postprocessing workstations, maximum intensity projections or volume-rendered 3D images can be reconstructed. Vessel tracking, automated segmentation methods, and color-coding techniques have further improved the quality of postprocessing capabilities. In contrast to epicardial vessels, demonstration of microcirculation and tissue perfusion requires sophisticated imaging techniques such as micro-CT to show microcirculation and perfusion CT, magnetic resonance (MR) imaging, or echocardiography to evaluate tissue perfusion. This chapter will place special emphasis on typical epicardial coronary anatomy as seen in coronary CTA. Myocardial microcirculation will be discussed briefly.



Epicardial Coronary Arteries



Embryology


It was suggested in earlier experiments that coronary vessels arise from the proepicardium, a transitory structure in the embryonic heart that forms epicardium and several internal tissues. 1 Recent histological analysis in mouse and cardiac organ culture has shown that coronary vessels arise from angiogenic sprouts of the sinus venosus, the major vein that returns circulating blood to the embryonic heart. 2 Some sprouting venous endothelial cells dedifferentiate into arteries and capillaries as they invade the myocardium and some remain on the surface and differentiate into veins. 2



Gross Anatomy


Coronary arteries are variable in anatomic origin, course, and branching. 3 In general, coronary vessels are named for the structures that they supply rather than for their origin. This nomenclature is based on the embryologic development pattern that the coronary vessel sprouts arise on the surface of the heart and only later connect to the aorta. Thus, various perturbations in the connections of the coronary vessels to each other and to the aorta occur subsequent to the formation of the coronary vessels. In spite of variability in coronary artery origin, course, and branching, the vast majority of individuals have two coronary arteries: the right coronary artery (RCA) and the left coronary artery. The left main (LM) coronary artery branches into the left anterior descending (LAD) and left circumflex (LCx) arteries (Fig. 18‑1). These three major coronary vessels are considered arteries, while the secondary ramifications are generally termed coronary branches. In spite of this generalization, it is not uncommon to call some small branches “artery,” that is, sinoatrial node (SAN) artery or posterior descending artery (PDA).


Instead of bifurcating into the LAD and LCx arteries, in 10 to 15% of patients, the LM trifurcates into the LAD, LCx, and ramus intermedius arteries 1 ,​ 3 (Fig. 18‑2). The ramus may arise directly from the LM artery (trifurcation) or near the orifice of the LAD or the LCx. Therefore, the ramus may actually be considered the first branch of the LAD or the LCx when arising from these vessels.


The relationship between the coronary artery branches at the crux cordis is called coronary artery dominance and is determined by the artery giving rise to the posterior interventricular branch/artery (PDA) 1 ,​ 3 ,​ 4 ,​ 5 ,​ 6 (Fig. 18‑3). The PDA and posterolateral (PL) branches that supply the inferior ventricular septum and the inferior wall, respectively, can originate from only the RCA (right dominant, 65–90%), from only the LCx artery (left dominant), or from both arteries (codominant). In CT studies, a “codominant pattern” is commonly described when the RCA gives off to the PDA, but all PL branches of the left ventricle (LV) and sometimes a second PDA arise from the LCx artery. The left-dominant system is more common in bicuspid aortic valve particularly when fusion is between right and left leaflets and may reach up to 48%. 7 Another definition of coronary dominance involves the artery that gives rise to the atrioventricular nodal (AVN) artery. The AVN artery arises from the RCA in almost 85% of cases.

Fig. 18.1 Coronary artery system. Color-coded volume-rendered CT angiography and invasive angiography (left and right injections) images in standard projections demonstrate normal coronary artery anatomy. Right anterior oblique (RAO), left anterior oblique (LAO) projections with caudal (CAU), or cranial (CRA) tilts are shown. The left main (LM) coronary artery divides into the left anterior descending (LAD) and the left circumflex (LCx) arteries. The LAD descends in the anterior interventricular groove and gives off diagonal (D) and septal (S) branches. In some cases, the LAD wraps around the apex to the inferior wall (as shown in these examples). The LCX moves in the left atrioventricular groove and gives rise to the left lateral atrial branch and one or more obtuse marginal (OM) and left posterolateral (PL) branches. The OM branches may be large and supply part of the anterior wall and most of the lateral wall of the left ventricle. The right coronary artery (RCA) gives origin to the conus and sinoatrial node arteries and descends into the right atrioventricular groove. Several right ventricular branches can arise from the descending section of the RCA. At the crux, the RCA then gives rise to a small atrioventricular nodal branch and then continues in the inferior (posterior) interventricular groove to provide the posterior descending (PD) branch. In many cases, the RCA travels beyond the crux and gives off PL branches to the posterior wall of the left ventricle.
Fig. 18.2 Coronary anatomy. Volume rendered CT images in anterior (Ant.), left anterior oblique (LAO), and apical projections. The anterior view shows the right coronary artery (RCA) descending in the right atrioventricular groove. A large conus (C) branch is seen arising from the aorta near the RCA ostium and is directed toward the right ventricular (RV) free wall. Marginal and acute marginal (AM) branches are usually seen in this projection. The distal RCA usually descends into the inferior (posterior) interventricular groove to provide the posterior descending (PD) branch. The PD branch can reach the apex and provide posterior septal branches. If the RCA extends beyond the crux, the branches are called posterolateral (PL) branches. The LAO view shows trifurcation of the left main coronary artery into the left anterior descending (LAD), the left circumflex (LCx), and ramus intermedius arteries. The ramus intermedius, also known as the intermediate branch or ramus, varies in length and size. It typically supplies the lateral wall of the left ventricle (LV) in a territory similar to that supplied by the diagonal (D) or obtuse marginal (OM) branches. The LAD descends in the anterior interventricular groove to reach the apex. During its entire course, the LAD gives rise to septal branches that penetrate into the interventricular septum; the first septal branch (S) is often large. The diagonal (D) branches are variable in size and number. The LCX enters the left atrioventricular groove and usually gives rise to one or several OM branches and may continue to end in several left PL branches (LCx dominant or codominant).
Fig. 18.3 Coronary dominance. Inferoposterior views of volume rendered CT angiographies. (a) In the right-dominant system, the right coronary artery (RCA) gives rise to the posterior descending (PD) branch and at least one posterolateral (PL) branch of the left ventricle. (b) In the left-dominant system, the left circumflex (LCx) artery is large and provides a PD branch. The RCA is small and does not extend beyond the acute margin of the heart (shown in inlay image). (c) In the codominant system, most of the PL branches are provided by the LCx and the PD branch by the RCA. LV, left ventricle; OM, obtuse marginal; RV, right ventricle.


Segmental Classification


Coronary anatomy varies, and several nomenclatures have been proposed to describe the angiographic anatomy of the coronary arteries. Currently, the most commonly used classification is based on three major coronary arteries with right-dominant, balanced, or left-dominant circulations. The extent of disease is usually defined as one-vessel, two-vessel, three-vessel, or LM disease. Significant stenotic disease is the presence of a stenosis of greater than 70% diameter reduction. 8


The American College of Cardiology and American Heart Association (AHA) Guidelines for invasive coronary angiography propose a topographic angiographic model that classifies the coronary arteries into 29 segments. 9 A more simplified classification based on an Ad Hoc Committee Report of the AHA is commonly used to interpret catheter and CT angiography studies 9 (Fig. 18‑4). Obviously, this model only shows a classic pattern of coronary vessel distribution and does not involve the variability that is frequently seen in the coronary artery branches.


Diagram of the topographic distribution of the coronary anatomy segments with description of the segments are shown in Fig. 18‑4. This diagram has been modified to include the right PL branch and the ramus branches, which were not shown in the original version of this classification.


For cross-sectional imaging, the standard 17–myocardial segment model of LV is suggested by the AHA for all cardiac imaging modalities to facilitate a more detailed analysis of regional left ventricular function and perfusion 10 (Fig. 18‑5). However, the adopted model shows significant variability and overlap of these segments assigned to a specific territory that makes it difficult to compare the results between different studies (Fig. 18‑6).


The vascular supply to the inferior wall of the LV has a spectrum of variants. It is mainly supplied by the RCA and the LCx. The LAD artery that wraps around the apex can supply some of the apical inferior wall (Fig. 18‑3). The lateral wall territory is supplied by the LCx in a vast majority of individuals but can be shared by a large diagonal from the LAD or the ramus branch (Fig. 18‑6). Myocardial blood supply in the inferolateral region corresponds to LCx or RCA distribution, and the inferoseptal region can be supplied by the LAD or RCA. 15 In a correlative MR study, the presence of hyperenhancement in the basal and mid-anterior and anteroseptal or apical anterior wall has been 100% specific for LAD artery occlusion. 14 The LAD infarct frequently involves the mid-anterolateral, apical lateral, and apical inferior walls. No segment is 100% specific for the RCA occlusion. A combination of hyperenhancement in the anterolateral and inferolateral walls is 100% specific for an LCx occlusion, and, when extended to the inferior wall, is also 100% specific for a dominant or codominant LCx occlusion. Hybrid imaging combined with anatomic information of vascular distribution provided by CTA is useful to study myocardial tissue perfusion abnormalities and the coronary anatomy for more accurate assignment to culprit vessels and to improve monitoring of targeted therapy. 11 CT studies have shown common overlap between LAD and RCA perfusion territories at the inferoseptal region and between LAD and LCx territories at the anterolateral region. It is important to realize that the LAD territory can be larger than the AHA-proposed 17-segment model, due to the involvement of the anterolateral and all the apical segments of the LV. 15

Fig. 18.4 Definition of coronary artery segments according to a report of the Ad Hoc Committee for Grading of Coronary Artery Disease, Council on Cardiovascular Surgery, American Heart Association. A simplified model is presented. (Used with permission from Austen et al. 9 )
Fig. 18.5 Long- and short-axis CT angiography images show the distribution of the left ventricular 17 myocardial segments based on the American Heart Association (AHA) model: basal anterior, 1 basal anteroseptal, 2 mid-anterior, 7 mid-anteroseptal, 8 apical anterior, 11 apical septal, 12 and apex 13 segments corresponded to LAD (red). Basal anterolateral, 6 basal inferolateral, 5 mid-anterolateral, 14 mid-inferolateral, 15 and apical lateral 16 segments correspond to LCX (yellow). Basal inferoseptal, 3 basal inferior, 4 mid-inferoseptal, 9 mid-inferior, 10 and apical inferior 17 segments correspond to RCA (green) distribution. ch, chamber; LAD, left anterior descending; LCx, left circumflex artery; RCA, right coronary artery. (Used with permission from Scanlon et al. 8 )
Fig. 18.6 (a) Short-axis CT angiography images show anatomical distribution of the coronary arteries in relation to the left ventricular segments. Arrows show territories of the LAD (red), RCA (blue), and LCx (yellow). (b) Maximal extent of myocardium perfusion for LAD (red), RCA (green and blue), and LCX (yellow) and overlapped regions in the polar map of 17 segments. D, diagonal branch; LAD, left anterior descending; OM, obtuse marginal branch of left circumflex artery (LCx); PD, posterior descending branch; PL, posterolateral; RCA, right coronary artery. 14


Common Variants



Left Coronary Artery

The LM coronary ostium is located in the midportion of the left sinus of Valsalva with a mean distance of 14 ± 3 mm from the annulus. It measures approximately 8 mm in diameter and travels for 5 to 15 mm before dividing into the LAD and LCx arteries. The mean bifurcation angle between the LAD and LCx is 80 ± 23 degrees 12 (Fig. 18‑7 , Fig. 18‑8 , Fig. 18‑9). Knowing the angle of bifurcation may be important for optimal stent deployment.


Anatomical variants of the LM are common. The ostium of the LM artery may arise at or above the sinotubular junction. When it arises at the sinotubular, the ostium may be small. A high origin the LM may be prone to accidental cross clamping or transection during aortotomy (Fig. 18‑10). A very low position ostium may become obstructive after percutaneous aortic valve implantation. A very short LM artery can cause technical problem during stenting of the LAD stenosis at its takeoff. Separate origins of the LAD and LCx in left sinus of Valsalva are another relatively common variants (Fig. 18‑11). The incidence is more common in bicuspid aortic valve, especially with left–right fusion and may reach up to 28%. 7


The LAD passes behind the main pulmonary artery to enter the anterior (superior) interventricular groove (sulcus) where it travels to reach the apex (Fig. 18‑7 , Fig. 18‑9). It gives rise to the perforating septal and one to three diagonal branches with viable sizes to supply the anterior two-thirds of the septum and the anterolateral wall of the LV including the superolateral papillary muscle, respectively (Fig. 18‑9). Small branches to the anterior wall of the right ventricle (RV) or the RV infundibulum may arise from the LAD. The distal segment of the LAD usually ends up near the apex, but it may wrap around it and travel further in the inferior interventricular groove. In rare cases, it may reach the cardiac crux (Fig. 18‑3 a).


A confusing variant of the LAD is called dual LAD. Type I dual LAD is worth mentioning because it is relatively common and may have clinical implications. In this variant, the LAD trunk is bifurcated (split LAD) into a short and a long branch to give off septal perforators. The short LAD stays in the proximal anterior interventricular groove branching into several septal perforators (usually 3–4; Fig. 18‑12). 17 This artery should not be confused with a large first perforating septal branch during catheter angiographies, which may need to be embolized for treatment of obstructive hypertrophic cardiomyopathy.


The LCx or circumflex artery arises from the LM coronary artery. It then passes under the left atrial appendage, to reach the left AV groove (Fig. 18‑7 , Fig. 18‑9). At this level, small atrial branches including a left SAN artery may arise from the LCx (Fig. 18‑13). The position of the LCx in relation to the AV groove is variable. The LCx gives off one or more obtuse marginal (left marginal) branches to supply the lateral and posteroinferior walls of the LV. It is not uncommon to see a small LCx to continue in the AV groove after giving off to a large obtuse marginal branch. The distal LCx can travel in the AV groove to reach cardiac crux. At the AV groove, it could be damaged during mitral valve surgical or percutaneous interventions. When dominant, the LCx gives rise to the posterior interventricular artery (Fig. 18‑3).


Myocardial bridging can be found in any epicardial artery, but greater than 60% occurs in the proximal LAD artery or one of diagonal branches (Fig. 18‑14). It refers to the intramyocardial course of a portion of a normally positioned epicardial coronary artery. The degree in which the coronary artery travels into the myocardium is variable in depth and length. The bridged segment is usually protected from atherosclerotic diseases and calcifications, but plaques can form proximal to it. 16 The significance of transient systolic narrowing (milking effect) of the bridged segment is not clear since most coronary blood flow occurs in diastole. The hemodynamic impact of myocardial bridging depends on the thickness and length of the bridge and rarely may be associated with exertional angina and acute coronary syndromes.

Fig. 18.7 Left coronary artery. Volume rendered CT images at 37-degree left anterior descending with 45-degree cranial tilt showing proximal coronary arteries before and after removal of the right ventricular outflow tract (RVOT) and the left atrial appendage (LAA). The left main (LM) is shown only after removal of the LAA. It measured 12 mm and the angle of bifurcation was 85 degrees. The LM and right coronary artery (RCA) ostia are located in mid to upper portion of the sinuses of Valsalva. D, diagonal; LAD, left anterior descending; LCx, left circumflex; LV, left ventricle; OM, obtuse marginal; R, right coronary sinus; RV, right ventricle.
Fig. 18.8 Early takeoff of the first diagonal (D1) branch. The location for ostial origin of the septal arteries is shown by red dots. The proximal segment of the LAD is defined from the origin to the takeoff of the first septal branch.
Fig. 18.9 Left coronary artery. Volume rendered CT images at 46-degree left anterior oblique with 53-degree cranial tilt showing proximal coronary arteries before (a, c) and after (b) removal of the right ventricular outflow tract (RVOT) and the left atrial appendage (LAA). The left main (LM) is shown after removal of the LAA and RVOT. It measured 21 mm. Ao, aorta; D, diagonal; LAD, left anterior descending; LCx, left circumflex; LV, left ventricle; OM, obtuse marginal; RV, right ventricle.
Fig. 18.10 High-riding right coronary artery (RCA) and left main (LM) artery. (a, b) Origin from the ascending aorta (AA). (c) Origin from the sinotubular junction.
Fig. 18.11 Combination of separate origin (arrow) of the left anterior descending (LAD) and left circumflex (LCx) arteries and a left-dominant coronary system. This combination is common in the bicuspid valve of the aorta. The right coronary artery (RCA) is small and nondominant. AA, ascending aorta; L, left coronary aortic sinus; LV, left ventricle; MPA, main pulmonary artery; OM, obtuse marginal; PL, posterolateral; RD, posterior descending; RV, right ventricle.
Fig. 18.12 Type 1 dual LAD. The dual LAD is defined and as an early bifurcation of the proximal LAD (the LAD proper) into a short and a long LAD. The short LAD stays in the proximal anterior interventricular groove (AIVG) and gives off most of the septal branches (white arrows). The long LAD travels to the apex, first along but outside the AIVG and gives off diagonal branches, then entering the AVIG to nourish the distal ventricular septum. LAO, left anterior descending; SAX, short axis.
Fig. 18.13 Left lateral atrial branch of the left circumflex (LCx) artery giving rise to the left sinoatrial node (SAN) artery. This artery is also called an S-shaped SAN artery because of its long tortuous anatomic course toward the superior vena cava (SVC) where the SAN is located. The artery passes between the left atrial appendage (LAA) and the left superior pulmonary vein (LSPV). Ao, aorta; D, diagonal; LAD, left anterior descending; LM, left main; LV, left ventricle; OM, obtuse marginal; RSPV, right superior pulmonary vein; RV, right ventricle.
Fig. 18.14 Myocardial bridging (arrows). Upper row: Myocardial bridging can be found in any epicardial artery but greater than 60% occurs in the proximal LAD artery. It may travel deep in the muscle and even reach the right ventricular (RV) cavity. Lower row: Myocardial bridge of the right coronary artery (RCA) next to the right ventricle (RV). Ao, aorta; LV, left ventricle.


Right Coronary Artery

The RCA arises from the right coronary sinus (Fig. 18‑15). Mean distance between the aortic annulus and the ostium of the RCA is 17 ± 3 mm. 13 There is remarkable variability in the branches of the proximal RCA or of the right aortic sinus close to the coronary ostium. The five arteries that may arise from the first 2 cm of the RCA include an anterior conal artery, a right SAN artery, an adventitial artery (vas vasorum) of the aortic root, a preventricular artery, and occasionally a right superior septal artery (right Kugel’s; Fig. 18‑16 , Fig. 18‑17). The conus, SAN, and superior septal arteries can arise with a separate ostium directly from the right coronary sinus, with the right conus artery being the most common variant. In its proximal course, the RCA enters the right AV groove and travels in it toward the posterior part of the interventricular groove to reach the crux of the heart. At this point or some time earlier, the RCA bifurcates into two branches: the posterior-descending branch and the PL branch(s) (Fig. 18‑3).


The PDA travels in the inferior (posterior) interventricular groove toward the apex of the heart. The right PL branch/branches supply the PL wall of the LV. These branches are usually small but can be large when the RCA is super-dominant and the LCx is diminutive (Fig. 18‑18). One or two marginal (acute marginal) branches arise from the mid-portion of the RCA to supply the RV free wall.


Variation in the course of middle and distal segments of the RCA is uncommon. A segment of the RCA may move outside the atrioventricular groove, dive into the myocardium (bridging; Fig. 18‑14), or enter the cavity of the right atrium (intracameral). 18

Fig. 18.15 Proximal right coronary artery (RCA). The RCA arises from the right coronary sinus behind the right ventricular outflow tract (RVOT) and enters the right atrioventricular groove and travels in it toward the posterior part of the interventricular groove. AA, ascending aorta; LCx, left circumflex artery; LV, left ventricle; RAA, right atrial appendage; RV, right ventricle.
Fig. 18.16 Proximal right coronary artery (RCA) branches. Five arteries that may arise from first 2 cm (ostial segment) of the RCA including an anterior conal artery, a right sinoatrial node (SNA) artery, an adventitial artery (vasorum) of the aortic root, a preventricular (marginal) artery, and occasionally a right superior septal artery (right Kugel’s). The right superior septal artery in this example is supplying the base of the ventricular septum. LV, left ventricle; RA, right atrium.
Fig. 18.17 Right coronary system branches. Most of these branches arise from ostial segment of the right coronary artery (RCA) or directly from the right coronary aortic sinus and may participate in conotruncal collateral circulation when a major artery is occluded. (a) Right anterior conus (direct origin from the aorta). It is the most common blood supply to the conal region. (b) Right sinoatrial node (SAN) artery (direct origin from the aorta) may provide branches to the atria and retroaortic circulation. Direct origin of the SAN artery (red arrows) can be seen in 1% of CT coronary studies. (c) Right superior septal artery (arrow) with direct origin from the aorta. This artery supplies the infundibular part of the basal ventricular septum and can participate in retroconal and retroaortic collateral circulations or connect with the first septal artery. It is seen in 3% of angiographic studies and 27% of heart dissections. (d, e) Right Kugel’s artery. Oblique short-axis and right ventricle (RV) long-axis views at the level of the central fibrous body showing extension of the right Kugel from the RCA along the right side of the aortic root to the interatrial septum. The right Kugel artery is probably a variant of the right superior septal artery, which courses toward the atrium and the atrioventricular nodal region rather than the base of ventricular septum. (f) Left atrial branch extending from the left main (LM) artery to the interatrial septum. This artery can extend to the SAN or participate in the retroaortic collaterals. LV, left ventricle; CS, coronary sinus; IVC, inferior vena cava; R, right coronary sinus; RV, right ventricle; RVOT, right ventricle outflow tract; S, septum; SVC, superior vena cava.
Fig. 18.18 Super-dominant right coronary artery (RCA) in a patient with single right main (RM) artery. The left main artery arises from the RM to supply the left anterior descending (LAD), left circumflex (LCx), diagonal (D), and obtuse marginal (OM) branches. Super-dominant RCA circles all the way to the left atrioventricular groove giving rise to the posterior descending (PD), posterolateral (PL), and OM branches.


Posterior Descending Artery

Evaluation of the anatomical origin and course of the PDA is important in angiographic studies of the coronary arterial system. The PDA determines the coronary dominance. In most patients, the PDA arises from the distal portion of the coronary artery, but in some cases an early takeoff of the PDA from the RCA is seen, which then courses toward the apex along the diaphragmatic surface of the RV. This common variant is referred to as “split RCA” 17 ,​ 19 (Fig. 18‑19). The splitting is usually proximal and leads to one branch with normal anatomical course in the right AV groove, giving rise to a PDA that supplies the proximal (basal) portion of the inferior interventricular groove, and a second branch that courses over the right ventricular free wall (acute marginal), and terminates in the distal part of the inferior interventricular groove as the distal PDA. 17 Other unusual variants include duplicated PDA or a single PDA that immediately bifurcates in two equal-sized branches (split PDA) running along the sides of the inferior interventricular groove (Fig. 18‑20). Split-RCA and dual-PDA cases appear to be benign and do not warrant a special management, except in the presence of atherosclerotic stenoses, which can make cannulation challenging during percutaneous coronary intervention. 20


In the codominant (balanced) coronary system, branches from both the LCx and RCA share vascular supply to the inferior interventricular septum. 5 ,​ 6 In other words, a “dual perfusion” exists that provides blood flow to both sides of the septum (Fig. 18‑3 , Fig. 18‑21). Therefore, using the perfusion concept, several vessel arrangements exist that can fit in the definition of a codominant coronary system. In patients presenting with an acute coronary syndrome, left coronary dominance appears to be associated with increased long-term mortality, especially with involvement of the inferior wall of the heart. 21 In most individuals with left coronary dominance, the RCA is usually a small and unreliable source of left myocardial perfusion.

Fig. 18.19 Split RCA or dual PDA. Upper row: Catheter and CT angiograms of the right coronary artery (RCA) in LAO projection (80 degrees with 15-degree cranial tilt). Lower row: Volume rendered views of the right lateral and inferior surface of the heart. Since most posterior septal perforators are supplied by the posterior descending artery (PDA), it has been described that in the presence of a dual PDA, the RCA may be called “dual” (similar to dual LAD variant). In this example, the short PDA (PDA2) ends quickly in the proximal inferior interventricular groove, while the long PDA (PDA2) travels over the right ventricle free wall to supply distal inferior septum. LAO, left anterior oblique; M, marginal; PL, posterolateral.
Fig. 18.20 Variants of the posterior descending artery (PDA). Left anterior oblique views of coronary angiograms in a patient with dual PDA (a) and one with a split PDA (b). Volume rendered views of the inferior surface of the heart showing dual (c) versus split (bifurcated) PDA (d) variants. In these examples, both PDA branches are equal in caliber and length running along sides of the inferior interventricular groove. AM, acute marginal branch; PDA, posterior descending artery; PL, posterolateral; RCA, right coronary artery.
Fig. 18.21 Codominant coronary system. Inferior and left lateral views of the heart. Using perfusion concept, several vessel arrangements exist that can fit in the definition of a codominant coronary system. In this example, the posterior descending (PD) artery has originated from the right coronary artery (RCA) and the posterolateral (PL) branch from the left circumflex (LCx). Both the RCA and the LCx reach the crux (arrow) of the heart. LV, left ventricle; RV, right ventricle.


Participants of Intercoronary Connections


Collaterals are interarterial connections that provide blood flow to a vascular territory whose original supply vessel is obstructed (acquired or congenital). The interarterial collateral connections are small with a wide range, between 40 and 200 µm, which may reach up to 1 mm. 22 The size of the majority of these connections is below the 0.2-mm spatial resolution of most angiographic imaging equipment. The spatial resolution of most current CT scanners is above 0.4 mm. In case a major artery becomes occluded (>90% stenosis), because of increased pressure through these small anastomotic vessels, they dilate and become visible by imaging methods.


Common sources for inter- and intracoronary collaterals include (1) peripheral branches of the right and left coronary arteries, (2) transseptal anterior and posterior septal perforators, (3) conotruncal anastomotic pathways, (5) distal LAD with PDA, (6) interatrial anastomotic pathways, and (7) intramyocardial pathways (Fig. 18‑22 , Fig. 18‑23).


The most common types of extracardiac aortocoronary collaterals are those between bronchial and coronary arteries and between the internal thoracic and coronary arteries 23 ,​ 24 (Fig. 18‑24). The connections are through the sites of pericardial reflections, which allow the collaterals to enter the subepicardial layer. Extracardiac collaterals are occasionally seen between the SAN artery and the mediastinal or bronchial arteries in chronic pulmonary and bronchial diseases (Fig. 18‑23).


Collateral flow may protect myocardium from transmural infarction and preserve wall function when a vessel is totally occluded. This can occur in congenital absence or acquired obstructions (Fig. 18‑23). Collateral after infarction usually occurs within weeks after occlusion or severe stenosis with a great inter-individual variability in the potential to recruit collateral vessels. 22 Collaterals will regress once the ischemic territory is revascularized. 25

Fig. 18.22 Intercoronary collaterals. Right marginal collateral (green arrows) connecting the right coronary artery (RCA) with the left coronary system (red arrow).
Fig. 18.23 Acquired conotruncal collaterals in chronic coronary artery disease. (a–c) Axial images in a patient with severe coronary artery disease. Extensive anastomotic collaterals are formed between the right and left coronary systems. (a) Preconal collateral between the conus artery and the left anterior descending (LAD). (b) Retroconal collaterals. (c, d) Retroaortic collaterals. (d) A different patient showing retroaortic collaterals through an enlarged left sinoatrial node (black arrows) artery supplying a diseased right upper lobe (chronic tuberculosis). AA, Ascending aorta; LA, left atrium; LAD, left anterior descending artery; MPA, main pulmonary artery; RCA, right coronary artery.


Interventricular Septum


Two groups of arteries are responsible for blood supply to the interventricular septum. These perforating septal branches are divided into the anterior and posterior septals. This nomenclature to define the anatomical position of these vessels is based on their relation within the heart. Within the thorax, however, the positions can more accurately be described as superior and inferior. 26


The anterior septal branches arise from the LAD and the posterior septal branches arise from the PDA (Fig. 18‑9 , Fig. 18‑12). They travel within the septum close to the right ventricular endocardium (Fig. 18‑12). The anterior perforating septal arteries are significantly larger and more numerous than their inferior counterparts. The first branch is the largest in 80% of cases. The number of superior septal arteries ranges from 1 to 10, with an average of 4.2, and the inferior one’s ranges from 0 to 7, with an average of 2. 26 The septal arteries are the basis of efficient intercoronary collateralization between the LAD and PDA and probably the most common type. 25


The first septal perforating artery is a branch of LAD artery that originates between 13 and 37 mm from the LAD ostium and has a diameter of 1 mm (Fig. 18‑25). The artery branches into three smaller ones. The main branch passes toward the medial papillary muscle and the other two passes into the septomarginal trabeculation. 26 When the medial papillary muscle is present, the first septal artery tends to pass toward the base of the medial papillary muscle. The artery supplies the base of the ventricular septum and the right ventricular side of the septum including the moderator band with branches to the conduction system including the bundle of His and proximal bundle branches. In hypertrophic cardiomyopathy, the artery is enlarged. Embolization of this artery is commonly performed as a way to treat left ventricular outflow tract obstruction seen in some cases of hypertrophic cardiomyopathy. 27 The source of the moderator band artery is one of the first three anterior septal arteries, with the second septal being the most common. This artery can extend to the anterior papillary muscle where it may anastomose with the marginal branches of the RCA. 28


The right superior descending (septal) artery is seen in 3 to 6% of angiography studies arising from the proximal RCA 29 ,​ 30 (Fig. 18‑25). It could be a potential intercoronary collateral connecting the proximal RCA to the distal RCA, LCx, or LAD through the atrial anastomotic network, retroaortic anastomotic pathway or trans septal connections (Fig. 18‑26; see the following sections).

Fig. 18.24 Color-coded volume-rendered CT images showing extracardiac aortocoronary collaterals (arrows) between the aortic arch and left coronary system supplying a prepulmonary fistula (purple) between the left coronary arteries and the main pulmonary artery (MPA).
Fig. 18.25 (a) Sagittal CT showing the first septal perforating artery of the left anterior descending artery (LAD). The septal perforating arteries originate at a right angle from the LAD and travel on the right side of the interventricular septum. The first septal artery (arrow) is usually the largest and varies in length from 2 to 5 cm. The inferior branches of the first septal perforator are directed toward the moderator band and supply the anterior papillary muscle of the tricuspid valve. Its superior branches supply the right bundle branch and the bundle of His. (b) Oblique coronal view showing the right superior septal artery (arrow). This artery supplies the infundibular septum and can participate in the retroaortic intercoronary collateral circulation. LV, left ventricle; RCA, right coronary artery; RV, right ventricle.
Fig. 18.26 Retroaortic intercoronary anastomotic pathway between the right superior septal artery (red arrow) and the left first septal artery (green arrow) in a patient with left main occlusion status post coronary artery bypass graft (CABG). RCA, right coronary artery; SANa, sinoatrial node artery.

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Mar 16, 2021 | Posted by in CARDIOVASCULAR IMAGING | Comments Off on 18 Coronary Arteries and Myocardial Perfusion
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